Low latency and high reliability have always been the key requirements for industrial applications such as factory automation, power and energy system monitoring and control, smart metering, and oil and gas businesses. In the past, such industrial control systems have included a large number of cables in order to connect, monitor and control the array of devices and sensor units across an industrial plant. It can be quite expensive to install, operate and maintain such a wired system. Despite these disadvantages, the industry is still reluctant to switch to cheaper wireless internet of things (IoT) solutions which have been on the market for the past few years.
The IEEE 802.15.4 standard specifies the physical layer and media access control (MAC) layer for low-rate wireless personal area networks, such as those used in industrial applications. Previous IEEE 802.15.4 standard solutions, whilst being cheaper, have not been able to deliver the desired low-latency and high reliability requirements due to their carrier sense multiple access with collision avoidance (CSMA/CA) based contention MAC access methods.
Time Division Multiple Access (TDMA) scheduling is an alternative to CSMA/CA. TDMA splits time into small intervals called timeslots. Each individual node in the network is assigned with a schedule (a set of timeslots), in which they can transmit data on a channel (e.g. a certain frequency bandwidth) without collision with other transmissions. Since TDMA is contention-less and collision free, it can achieve much lower multi-hop latency and higher communication reliability compared with CSMA/CA based approaches. Having said this, it still suffers from multi-path fading and interference from other devices in the Industrial, Scientific and Medical Radio (ISM) band such as WiFi, Bluetooth and microwave devices, to name a few.
To overcome these limitations, the Time-Slotted Channel Hopping (TSCH) MAC provides a promising way to enable deterministic mesh networking and paves the way for future low-power wireless industrial applications (the Internet of Important things). TSCH achieves improved communication reliability via channel hopping and avoids external interference operating at the same frequency band. Channel hopping helps to spread the risk of collisions by periodically switching the channel over which transmissions are made. TSCH also maintains low radio duty cycle and low energy consumption by synchronizing network nodes.
The Internet Protocol version 6 over the TSCH MAC (6TiSCH) standard is a developing Internet Engineering Task Force (IETF) standard. Along with a scheduling mechanism to enable deterministic wireless communication, it can provide ultra-low latency and high reliability guarantee for industrial applications.
Although the current 6TiSCH standard defines the mechanism of how the TSCH MAC executes a TDMA schedule in a network, it does not specify how to build an optimized schedule. It is not an easy task to find an optimal schedule in multi-hop, low power wireless networks.
Industrial applications can have varying latency demands. For example, process control or monitoring applications (e.g. environmental monitoring) can be non-time critical. On the other hand, factory automation, disaster defence and safety applications can be highly sensitive to delays. Thus, the latency and system response time of such applications is important and the Quality of Service (QoS) requirements are often in the scale of hundreds of milliseconds.
In addition, wireless network nodes are often powered by batteries with limited capacity. As such, power efficiency should be maximised and unnecessary power usage within devices should be minimised.